Offshore platform structures

ABSTRACT

This invention relates to the construction of offshore platforms on selected sites on the sea floor. The method includes transporting to the site at least one center column and at least three batter columns, erecting the center column so that its upper end extends above the water&#39;&#39;s surface and its lower end is fixedly secured to the sea floor, and then erecting the batter columns around the center column in bracing relation to the center column and to each other thereby forming a support structure, and constructing a working platform on the erected support structure.

United States Patent Gibson et a1.

OFFSHORE PLATFORM STRUCTURES Inventors: Robert G. Gibson; Samuel C.

Carruba, both of Houston, Tex.

Assignee: Fluor Corporation, Los Angeles,

Calif.

Filed: May 4, 1973 Appl. No.: 357,446

US. Cl. 6l/46.5 Int. Cl E02b 17/00 Field of Search 61/46.5, 46, 50, 52;

References Cited UNITED STATES PATENTS Lacy 61/46.5

Wilson 61/46.5 Harris 61/46 Dec. 10, 1974 9/1959 l-lutchison ..6l/466/1960 Walker ..6l/46.5

Primary Examiner.lacob Shapiro Attorney, Agent, or FirmMichael P.Breston [5 7] ABSTRACT This invention relates to the construction ofoffshore platforms on selected sites on the sea floor. The methodincludes transporting to the site at least one center column and atleast three batter columns, erecting the center column so that its upperend extends above the waters surface and its lower end is fixedlysecured to the sea floor, and then erecting the batter columns aroundthe center column in bracing relation to the center column and to eachother thereby forming a support structure, and constructing a workingplatform on the erected support structure.

6 Claims, 21 Drawing Figures SHEET 10F 4 PATENTED DEC 10 I974 FIG. 2.

sumanra MTENTED DEC 10 I974 pmmgnuzc 10 m2: SHEEI 30F 4 3,852,969

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OFFSHORE PLATFORM STRUCTURES BACKGROUND OF THE INVENTION During theearly stages of offshore construction, limitations were imposed bythe'available equipment for handling large structural members. Mostplatform structures were erected in shallow waters say between to 65feet. As a fundamental substructure there was used a four-leggedtemplate or jacket made of tubular legs reinforced by horizontal anddiagonal braces. The

, jackets were fabricated in onshore fabrication plants .would beconstructed over the jackets. The known method had two chief drawbacks:it required several jackets and an excessive amount of onsiteconstruction.

Efforts were made to greatly increase the lifting capacity of the craneson the derrick barges and to improve the methods for manipulatingrelatively large substructures on barges as well as on the offshoreerection site. These increased handling capabilities allowed the use ofrelatively large jackets so that the number of jackets required tosupport a given area of the working platform could be reduced. However,the reduction in the number of required jackets was accompanied by anincrease in the numberof tubular legs per jacket and in their diameters.

Asthe search for oil moved to deeper waters, say between 65 to 3 hundredfeet, the weight handling capabilities of the cranes on existing derrickbarges again became limiting factors in the construction of offshoreplatforms using jackets as the basic support frame. These limitingfactors prompted the rapid development of jacket-launching techniquesfrom the decks of barges into the water.

There is now a demand for structures positioned in water depths greaterthan 300 feet. The conventional jacket-type structures now in use inshallow waters are not well suited for such depths because they would bedifficult to tow to the construction site, they would not frame enoughpiles to provide lateral support, and the required wall thickness fortheir standard members would make it difficult for them to float or tobe launched from the decks of barges.

It has been proposed for such great depths to construct jacket-typestructures having great diameter legs that would provide flotation. Butsuch large structures require special fabrication facilities which arenot readily available. Such facilities would have to be located onshoreto minimize problems of transporting such large substructures over landwaterways and under existing bridges. To allow the jackets to float, thelarge-diameter legs of the jackets would have to be constructed ofrelatively thin walls, relative to their diameters, and they wouldrequire considerable stiffening to protect them against collapse byhydrostatic pressure. The very large-diameter tubular legs have theinherent disadvantage in that they present an excessive planar area towinds and storms and hence are subject to excessive lateral loads. Toresist such lateral loads, a great number of piles are required toground the legs of the jackets to the sea floor. Yet, even though thejackets are constructed of relatively large diameter legs, they may notbe able to frame the required number of piles. Finally, the erectedplatform structure would still be relatively limited in its overalldimensions at its base, a fact which tends to reduce the lateralstability of the entire platform structure.

Accordingly, it is a main object of the present invention to overcomemany of the above-described and other apparent drawbacks of knownoffshore platform construction methods.

In particular, it is an object of the present invention to provide amethod of constructing an offshore platform which requires substructureswhich can be fabricated within inland fabrication yards, which can betransported on inland wateways with existing transportation barges,which require relatively small-diameter legs, and which require areduced number of piles per leg.

SUMMARY OF THE INVENTION A method of constructing an offshore platformon a selected site on the sea floor by transporting to the site at leastone center column having a length which is greater than the depth of thebody of water at the site, also transporting to the site at least threebatter columns each having a length which is considerably greater thanthe length of the center column, first erecting the center column on aselected area of the site so that its upper end extends above the waterssurface and its lower end is fixedly secured to the sea floor, thenerecting the batter columns around the center column so that their upperends are secured to angularly spaced portions about the upper end of thecenter column and their lower ends are fixedly secured to the sea floorin bracing relation to the center column and to each other therebyforming a support structure, and thereafter constructing a workingplatform on the erected support structure.

In one method aspect of this invention, the top ends of the battercolumns are pivotably secured to the upper end of the center columnwhile they are substantially horizontally maintained on the watersurface and thereafter the free ends of the batter columns are graduallylowered to the sea floor toward selected points on the constructionsite.

In another method aspect of the invention, the batter columns are firstvertically erected around the uprighted center column and then theirupper ends are moved toward and secured to the upper end of the centercolumn.

In both method aspects, securing means such as piles are driven throughthe legs of the center column and of the batter columns as required tomaintain the support structure fixed to the sea floor. By removing suchsecuring means, the platform structure can be disassembled and moved forerection to another construction site.

The method of the present invention lends itself advantageously forconstructing a platform structure over an existing offshore well siteover which the center column is required to be erected. The centercolumn will allow the servicing of a plurality of wells say up to two orthree dozen wells. The center column is used as a reference for theerection of the batter columns.

The platform structures constructed in accordance with the presentinvention are characterized by lateral stability of the workingplatform. This advantage is of great significance since any appreciablelateral displacement of the platform becomes highly objectionable, forexample, ifa drilling platform were to become displaced too far from itsposition above the wellbore, then the equipment interconnecting thedrilling platform with the wellbore could become severally damaged orperhaps completely destroyed.

Since the batter columns are widely spaced-apart at their lower ends,making the overall base dimensions of the erected support structurerelatively large, the axial load on the piles can resist a great portionof the lateral load on the platform structure resulting from windstorms.

The platform structures in accordance with the method of the inventionare cheaper to manufacture, require less onsite fabrication, and arerelatively lightweight, thereby achieving considerable savings in theamount of steel required.

Also, since platform structures periodically become subjected tohurricane force winds which produce great fluctuations in the depth ofthe water body above the marine floor, the platforms are generallyconservatively elevated above the highest expected water surface underthem. The platform structures constructed in accordance with the methodof the present invention are particularly well adapted, with theirwidely spacedapart batter legs, to support such highly elevatedplatforms.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic planrepresentation of one illustration of an embodiment of a platformstructure assembly constructed in accordance with the method of thisinvention and adapted to support a working platform;

FIG. 2 is a schematic sectional representation on line 2-2 in FIG. 1;

FIGS. 3 and 4 are sectional views taken on lines 3-3 and 4-4,respectively, in FIG. 2;

FIGS. 5-8 illustrate the method steps of transporting the center columnto and uplifting it on the selected offshore site;

FIGS. 9-12 illustrate one method aspect of the invention fortransporting the batter columns and horizontally maintaining them in thebody of water while their top ends are being pivotably secured toangularly spaced-apart points on the upper end of the center column.Thereafter, the free ends of the batter columns are gradually lowered tothe marine bottom;

FIGS. 13 and 14 illustrate the manner of positioning a working platformon the erected support structure;

FIGS. 15 and 16 illustrate steps which may be used for securing the legsof the support structure to the marine bottom;

FIGS. 17-21 illustrate another method aspect of the invention whereinFIG. 17 illustrates the launching of the batter columns to theconstruction site;

FIG. 18 illustrates the uprighting ofa batter column;

FIG. 19 illustrates a method of moving the upper end of each battercolumn toward the upper end of the center column; and

FIGS. 20 and 21 illustrate the completion steps of the platfromstructure.

Generally, the invention relates to methods of constructing fromcolumnar member's fixed offshore work- 'ing platforms such as are usedfor supporting equipment and supplies, as in drilling for or producingoil and gas wells. Each column is preferably made in an onshorefabrication plant and each is typically provided with buoyancycontrolling compartments and with conventional means (not shown) forcontrollably ballasting and deballasting the column. Such means are sogenerally employed in the offshore construction art that they need notbe described herein, and any such widely used and known means or theirequivalents may be employed to move the lower end of each column towardor away from the marine bottom.

Referring now more specifically to the drawings and in particular toFIGS. l-4 thereof, there is shown a platform structure generallydesignated as 10, erected from onshore-fabricated columns in a body ofwater having a floor 13 and a water line 15. Platform structure 10 isshown as comprising an upright center column 12 positioned in the bodyof water and being adapted to angularly receive a plurality of batter orstabilizing columns 16 (FIGS. 1 and 2). Center column 12 ordinarily hasa height when landed on the marine floor 13 such as to project its upperend 14 slightly above the water line 15. The upper end of center column12 is provided with an enlarged linking frame 20 adapted to receive theupper ends of a plurality of batter columns 16. The terms upper andlower ends when referring to a particular column are with reference tothe uprighted position of the column in the body of water.

The minimum numbers of columns 12 and columns 16 required are one andthree, respectively, but a greater number of each of such columns 12 or16 may be employed. In the embodiment shown in FIG. 1, four such columns16 are illustrated as being symmetrically and angularly positionedrelative to the linking upper frame 20. Each batter column may beprovided at its lower end with a suitable grounding leg 18 which, in itssimplest form, may be a large can, as better illustrated in FIGS. 15-16.

When the center column 12 is grounded after landing on the marine bottomand the batter columns 16 are attached thereto in bracing relationship,they form a support structure 10 adapted to receive on the upper endthereof a prefabricated transition structure 21 having a plurality oflegs 22 arranged to mesh and longitudinally align with the center column12 and with the batter columns 16. The transition frame 21 is adapted tosupport the desired working platform 24 which typically carriesequipment and supplies for drilling or producing oil and gas wells, orfor other purposes as may be required.

FIG. 3 illustrates a center column 12 which may comprise a plurality ofrelatively large-diameter, long, tubular legs 26 which can be used todefine ballasting and deballasting compartments (not shown) forcontrollably turning the center column in the body of water. Thevertical legs 26 are interconnected by cross-bracing tubular members 27which also can define fluid compartments in fluid communication with thecompartments in the legs 26. When the center column 12 is used in aplatform structure adapted for drilling operations, it is conventionalto provide inside column 12 a plurality of conductor guides 28interlinked by cross-bracing members 29. The assembly of conductorguides 28 is framed to the vertical legs 26 by members 30. Conductorpipes are then installed through the guides after the support structureis erected.

While the center column 12 is shown as having a rectangularconfiguration, it could have a triangular plan configuration, and whileonly four legs 26 are shown more than four legs would normally beemployed. Also, to save material, each batter column 16 may have atriangular configuration, as shown, constructed from three long hollowtubular legs 26 interconnected by bracing members 27 (FIG. 4). Thenumber of hollow tubular legs 26 could of course vary and each wouldnormally also define fluid compartments (not shown) for selectively andcontrollably ballasting the same with conventional means (not shown).The bracing framework 27 is constructed to provide maximum rigidity tothe batter columns.

The rate of ballasting of the center column 12 and of each batter column16 can be controlled so that the lower end of each column slowly sinksrelative to its upper end, thereby uplifting the column toward anupright or inclined position in the body of water. Ballasting of eachcolumn is controlled from the water surface and may be continued untilthe column rests on and is firmly grounded in the marine floor 13. Itwill be understood that by removing the grounding means, each columncould be tilted back to its horizontal floating position in the waterbody.

The method aspects of the invention, used in constructing the platformstructure 10 shown in FIGS. 1-4, will now be illustrated with referenceto FIGS. 5-16 and FIGS. 17-21.

FIGS. 5-8 illustrate the preferred steps used to erect the center column12 and fixedly ground it to the marine bottom 13. In FIG. 5 the centercolumn 12 is shown as being transported on a cargo barge 32 and carriedto the selected offshore site. At the construction site, center column12 is removed from the deck of barge 32 and allowed to float in the bodyof water. A crane 33 of a crane barge 34 is connected to the upper endof column 12 (FIG. 6). The column is then selectively and controllablyballasted to cause it to tilt (FIG. 7) slowly until the lower endthereof lands precisely on the desired point within the site which couldbe a marked well head on the marine bottom 13. Crane 33 will then assistto upright column 12. While maintaining the center column 12 in such anupright position, a pile driver 35 (FIG. 8) will drive piles 36 throughthe columns tubular legs 26 into the marine bottom 13 thereby securelygrounding the center column l2 thereto. The piles could be removed, ifand when desirable, and the process reversed to bring column 12 back toits original horizontal position.

A method of installing the first batter column 16 is illustrated withreference to FIGS. 9-12. The same method could be used to install theremaining batter columns 16.

After transporting column 16 on a cargo barge 32 or by floating it tothe construction site, the upper end thereof is hingedly secured to thelinking frame by suitable pivot means 37 while the column 16 remains onthe deck of the barge 32 (FIG. 9) The lower end of column 16 is thenraised upwardly by crane 33 (FIG. 10) to allow the cargo barge 32 tomove out from under column 16. Thereafter the lower end of column 16 islowered into the body of water and selectively and controllablyballasted until its leg 18 lands at a desired location on the marinebottom 13. It will be appreciated that column 16 can float in the bodyof water so that the assistance of crane 33 in the process of tiltingcolumn 16 can be reduced to a minimum.

The length of column 16 is considerably greater than the length ofcolumn 12 and hence than the maximum anticipated depth of the body ofwater near center column 12. Therefore, leg 18 of column 16 lands at apoint which is at a considerable distance away from the grounded end ofcenter column 12. Depending on the depth of the water, this distance maybe on the order of several hundred feet.

It will be appreciated that in the platform structure constructed inaccordance with the present invention, the lower ends of the battercolumns 16 are not required to be secured to any bottom base member suchas was conventional with completely prefabricated platform assembliesmoved to the desired offshore location and grounded to the marine bottomeither fixedly or removably.

The grounding of can 18 for clay and sand bottoms can be accomplished byfirst jetting a cavity 45 in the marine bottom 13 with the aid of aservice vessel 38 (FIG. 11). It has an umbilical cable 40 within whichare housed a plurality of flexible conduits for supplying jet streams 42to a plurality of jetting hoses 43, each having an outlet 44 (FIG. 16).The jetting action will remove the soil around can 18 to allow it tobecome buried completely in cavity 45. After can 18 is completely forcedinto the marine bottom 13 (FIGS. 15-16), piles 36 are then driven by apile driver 50 (FIG. 12) through the tubular legs 26' and can 18 ofcolumn 16. In this fashion, can 18 is now firmly grounded to the marinebottom.

For hard soils, known digging methods including jetting could beemployed to ground cans 18, as will be apparent to those skilled in theart.

The batter column 16 will be inclined at an angle 0 relative to thecenter vertical column 12 (FIG. 11). If this angle 6 is too small, theremay be a need to employ not a pile driver 50 for. hammering the piles 36into the marine bottom 13, but a combination of driving and drilling(not shown) to install the piles 36 into the marine bottom. After pilesare installed through the tubular legs 26' of column 16, they aresubsequently welded to the top ends thereof in conventional manner.

After installing the first column 16, a second such column is thenpivotably connected to the pivot means 37 on the linking frame 20 andballasted as above described until its leg 18 lands on a desired pointon the marine floor 13. The landing point can be easily determined sincethe angle 0 of the second column 16 with reference to the centervertical column 12 will generally be the same as that of the firstcolumn 16. Leg 18 of the second batter column 16 is then grounded asabove described.

The third and fourth columns 16 are then installed in succession in thesame manner, thereby completing the erection of the support substructure10'.

The method of installing the working platform is illustrated withreference to FIGS. 13-14. An onshore fabricated intermediate structure21 is transported to the site and lowered by crane 33 in position on topof the primary substructure 10 so that its legs 22 mesh and become inlongitudinal alignment with the upper ends of the center and battercolumns. After being so positioned, the intermediate structure 21 isfully structurally integrated with the primary structure 10 and, ifdesired, the pivots 37 are welded off.

To complete the installation, the working platform 24 itself, which maybe of unitary or sectionalized construction. is now lowered by crane 33on top of and framed to the intermediate structure 21. Platform 24 isnow ready to receive equipments and supplies.

In another aspect of the invention the erection of the platfromstructure can be carried out as illustrated in FIGS. l72l. The erectionof the center column 12 will be carried out in the manner previouslydescribed in connection with FIGS. -8. A batter column 16a is launchedand brought to the construction site. Each batter column includesbetween its ends a buoyancy chamber 60 and a large can 180 at the lowerend thereof. The tilting of each batter column 16a is accomplishedsubstantially in the manner previously described in connection with thetilting of first batter column 16 (FIGS. 9-12).

Whereas in the first aspect of the invention, the upper ends of thebatter columns are first consecutively pivotably secured to the upperend of the center column and then the lower ends of the batter columnsare gradually tilted toward their preselected positions on the marinebottom 13, in the second aspect of the invention, the batter columns arefirst erected in a substantially upright position at their selectedpoints around the center column 12 and then their upper ends are movedtoward the center column, as by a winch 61 having a rope 63 attached toa ring 62 on each batter column 16a, and/or with the assistance of cranebarges.

After thusly inclining all batter columns toward the center column,their upper ends are welded securely to the linking frame 20 on thecenter column, as previously described in connection with the firstaspect of the invention, thereby completing the erection of the supportstructure All columns will again have their lower ends grounded to themarinebottom by piles 36. The completion of the construction jobis-again accomplished by positioning on top of the support structure 10'an intermediate structure 21 and framing thereto the working platform24.

While this invention has been illustrated in connection with preferredembodiments thereof, it will be apparent that various modifications arepossible all falling within the scope of the claims attached hereto:

What is claimed is:

1. A method of constructing an offshore platform structure on a selectedsite on the sea floor, comprising:

transporting to the site at least one center column having a lengthwhich is greater than the depth of the body of water at said site;

transporting to said site at least three batter columns, .the centralcolumn and each batter column having a multi-legged structure consistingof at least three legs, each leg having an upper end, a lower end, andeach batter column having a length which is considerably, greater thanthe length of said center column;

erecting the center column on a selected area within said site wherebyits upper end extends above the water surface;

securing the lower end of said center column to said floor;

erecting the batter columns around said center column, welding the upperends of the batter columns to angularly spaced portions about the upperend of the center column above the water surface and fixedly securingtheir lower ends to said floor in bracing relation to said center columnand to each other thereby forming a support structure; driving pilesthrough the entire length of certain ones of the legs in each column tothereby fixedly secure each column to the sea floor, whereby the axialload on each pile resists a substantial portion of the lateral load onsaid support structure; and

framing a working platform onto the erected support structure.

2. The method of claim 1 wherein the erecting step of said battercolumns includes:

. first pivotably securing the upper end of each batter column to saidcenter column while maintaining it in a substantially horizontalposition within the body of water, and

gradually tilting each batter column toward its selected position onsaid site.

3. The method of claim 1 wherein said erecting step includes of saidbatter columns:

first positioning the lower ends of said batter columns on theirselected positions on the floor of said site, and

subsequently moving the upper ends of said batter columns toward saidcenter column.

4. A method of constructing an offshore platform structure positioned ina body of water and extending from the bottom thereof to support aworking platform above the waters surface, comprising:

erecting a vertical center column in the body of water having pivotmeans at angularly spaced apart points on the upper end of said column;

pivotally securing a plurality of batter columns to said pivot meanswhile maintaining said batter columns in a substantially horizontalposition, each batter column having a multi-legged structure consistingof at least three legs, each leg having an upper end, a lower end, and alength which is considerably greater than the length of said centercolumn;

controllably ballasting the batter columns until their lower ends landon the marine bottom;

affixing the lower ends of all batter columns to the marine bottom inbracing relation to the center column and to each other, whereby saidcenter column and said batter columns form a support structure;

driving piles through the entire length of certain ones of the legs ineach batter column to thereby fixedly secure each column to the seafloor, whereby the axial load on each pile resists a substantial portionof the lateral load on said platform structure; and

framing onto said support structure a working platform.

5. An offshore platform structure comprising:

at least one upright center column extending above the water surface;

at least three batter columns, each column having a multi-leggedstructure consisting of at least three legs, each leg having an upperend, a lower end, and each having a length which is considerably greaterthan the length of the center column to provide a large batter anglerelative to the vertical, the upper ends of the batter columns beingwelded above the water surface to angularly spaced portions about theupper end of the center column and the lower ends of the batter columnsbeing secured to the marine bottom in bracing relation to the centercolumn and to each other;

plurality of driven piles through the entire length of certain ones ofthe legs in each column to thereby fixedly secure each column to the seafloor, whereby the axial load on each pile resists a substantial portionof the lateral load on said platform structure; and

a working platform framed to the upper ends of said center column andbatter columns.

6. A method of constructing an offshore platform structure extendingover a selected site on the sea floor, comprising:

transporting to and erecting on said site a central column having alength which is greater than three hundred feet;

transporting to said site at least three batter columns,

each batter column having a multi-legged structure consisting of atleast three legs, each leg having an upper end, a lower end, and alength which is considerably greater than the central column;

uprighting each batter column by pivotally securing it above the watersurface to the upper end of the central column and by ballasting itslower end, whereby the lower ends of said batter columns are widelyspaced apart;

grounding the lower end of each uprighted batter column;

welding above the water surface the upper ends of said batter columns tosaid center column;

driving piles through the entire length of certain ones of the legs ineach batter column to thereby fixedly secure each column to the seafloor, whereby the axial load on each pile resists a substantial portionof the lateral load on said platform structure;

lowering a transition structure having a plurality of legs over thewelded upper ends of said columns;

structure.

1. A method of constructing an offshore platform structure on a selectedsite on the sea floor, comprising: transporting to the site at least onecenter column having a length which is greater than the depth of thebodY of water at said site; transporting to said site at least threebatter columns, the central column and each batter column having amulti-legged structure consisting of at least three legs, each leghaving an upper end, a lower end, and each batter column having a lengthwhich is considerably greater than the length of said center column;erecting the center column on a selected area within said site wherebyits upper end extends above the water surface; securing the lower end ofsaid center column to said floor; erecting the batter columns aroundsaid center column, welding the upper ends of the batter columns toangularly spaced portions about the upper end of the center column abovethe water surface and fixedly securing their lower ends to said floor inbracing relation to said center column and to each other thereby forminga support structure; driving piles through the entire length of certainones of the legs in each column to thereby fixedly secure each column tothe sea floor, whereby the axial load on each pile resists a substantialportion of the lateral load on said support structure; and framing aworking platform onto the erected support structure.
 2. The method ofclaim 1 wherein the erecting step of said batter columns includes: firstpivotably securing the upper end of each batter column to said centercolumn while maintaining it in a substantially horizontal positionwithin the body of water, and gradually tilting each batter columntoward its selected position on said site.
 3. The method of claim 1wherein said erecting step includes of said batter columns: firstpositioning the lower ends of said batter columns on their selectedpositions on the floor of said site, and subsequently moving the upperends of said batter columns toward said center column.
 4. A method ofconstructing an offshore platform structure positioned in a body ofwater and extending from the bottom thereof to support a workingplatform above the water''s surface, comprising: erecting a verticalcenter column in the body of water having pivot means at angularlyspaced apart points on the upper end of said column; pivotally securinga plurality of batter columns to said pivot means while maintaining saidbatter columns in a substantially horizontal position, each battercolumn having a multi-legged structure consisting of at least threelegs, each leg having an upper end, a lower end, and a length which isconsiderably greater than the length of said center column; controllablyballasting the batter columns until their lower ends land on the marinebottom; affixing the lower ends of all batter columns to the marinebottom in bracing relation to the center column and to each other,whereby said center column and said batter columns form a supportstructure; driving piles through the entire length of certain ones ofthe legs in each batter column to thereby fixedly secure each column tothe sea floor, whereby the axial load on each pile resists a substantialportion of the lateral load on said platform structure; and framing ontosaid support structure a working platform.
 5. An offshore platformstructure comprising: at least one upright center column extending abovethe water surface; at least three batter columns, each column having amulti-legged structure consisting of at least three legs, each leghaving an upper end, a lower end, and each having a length which isconsiderably greater than the length of the center column to provide alarge batter angle relative to the vertical, the upper ends of thebatter columns being welded above the water surface to angularly spacedportions about the upper end of the center column and the lower ends ofthe batter columns being secured to the marine bottom in bracingrelation to the center column and to each other; a plurality of drivenpiles through the entire length of certain ones of the legs in eachcolumn to thereby fixedly secure eacH column to the sea floor, wherebythe axial load on each pile resists a substantial portion of the lateralload on said platform structure; and a working platform framed to theupper ends of said center column and batter columns.
 6. A method ofconstructing an offshore platform structure extending over a selectedsite on the sea floor, comprising: transporting to and erecting on saidsite a central column having a length which is greater than threehundred feet; transporting to said site at least three batter columns,each batter column having a multi-legged structure consisting of atleast three legs, each leg having an upper end, a lower end, and alength which is considerably greater than the central column; uprightingeach batter column by pivotally securing it above the water surface tothe upper end of the central column and by ballasting its lower end,whereby the lower ends of said batter columns are widely spaced apart;grounding the lower end of each uprighted batter column; welding abovethe water surface the upper ends of said batter columns to said centercolumn; driving piles through the entire length of certain ones of thelegs in each batter column to thereby fixedly secure each column to thesea floor, whereby the axial load on each pile resists a substantialportion of the lateral load on said platform structure; lowering atransition structure having a plurality of legs over the welded upperends of said columns; welding the legs of said transition structure tothe welded upper ends of said columns; and framing a working platformonto said transition structure.